\documentclass{article}


%%-----------PGF/TIKZ
\usepackage{pgf}
\usepackage{tikz}
\usetikzlibrary{arrows,shapes,shapes.arrows,positioning}

\usepackage{palatino}

% Use preview.sty to crop the page
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{pgfpicture}
\setlength\PreviewBorder{0pt}

\begin{document}

\begin{figure}
\begin{tikzpicture}
    [
    node distance=8mm and 20mm,
    scale=.8,
    every node/.style={
    	draw=black,
	    thick,
        text width=15em,
        top color=gray!20, 
        bottom color=white,
        text centered 
    },
    block/.style ={rectangle},
    decision/.style = {
        diamond,
        shape aspect=2,
        inner sep=-8pt,
        text badly centered 
    },
    trap/.style = {
        trapezium,
        trapezium left angle=60, 
        trapezium right angle=120
    },
    ]

	\node(begin) [block] {Begin with initial conditions at time interval $k=1$};
    \node(assume) [trap,below=of begin] {Given conditions at time $k$, calculate flow across each control structure, assuming zero frictional losses};
    \node(rk) [trap,below=of assume] {Calculate the resulting conditions at time $k+1$ (Runge Kutta routine)};
    \node(endsim) [decision,below=of rk] {End of Simulation?};
    \node(friction) [trap,right=of begin] {Compute frictional losses resulting from the estimated flow};
    \node(flow) [trap,below=of friction] {Compute the estimated flow rate, given the effect of frictional losses};
    \node(converge) [decision,below=of flow] {Flow rate converges?};
    \node(end) [block,right=of endsim,below=of converge] {End Simulation};

   
    
    \path[draw,-stealth'] (begin) -- (assume);
    \path[draw,-stealth'] (assume) edge [out=0,in=180] (friction);
    \path[draw,-stealth'] (endsim) edge [out=180,in=180] (assume);
    \path[draw,-stealth'] (rk) -- (endsim);
    \path[draw,-stealth'] (endsim) edge [out=0,in=180] (end);
    \path[draw,-stealth'] (friction) -- (flow);
    \path[draw,-stealth'] (flow) edge [out=270,in=90] (converge);
    \path[draw,-stealth'] (converge) edge [out=270,in=90] (end);
    \path[draw,-stealth'] (converge) edge (rk);
    \path[draw,-stealth'] (converge) edge [out=0,in=0] (friction);


	
\end{tikzpicture}
\end{figure}
\end{document}
